Sulphur based proppants and process therefor

ABSTRACT

A proppant for enhancing the recovery of petroleum from rock formations comprises pellets of sulphur and a binding and strengthening additive, preferably comprising cement. A process for manufacturing the proppant comprises first melting sulphur to form a liquid and forming a mixture of the liquid sulphur with cement powder. The mixture is then cooled and pelletized to form pellets of a sulphur and cement composite. The pellets are then dried and sorted by size.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to proppants used in fracturingprocesses for petroleum extraction. More specifically, the inventionrelates to proppants made principally from sulphur and to methods forproducing such proppants.

[0003] 2. Description of the Prior Art

[0004] In the field of petroleum extraction from subterranean reservoirsby means of drilled wells, it is often found that reservoirs having lowfluid permeability result in wells that have low production rates. As asolution to this problem, hydraulic fracturing has become widely used asa method of enhancing production from petroleum bearing rock formations(reservoirs). Generally, fracturing involves pumping a viscous fluid,under high pressure, into the reservoir so as to form fractures in therock. Such fractures serve to increase the permeability of the reservoirthereby leading to increased petroleum production rates.

[0005] In the hydraulic fracturing process, it is known to utilizeproppants that are mixed into the viscous fluid to form a slurry orsuspension. Using high pressure hydraulic pumps, the slurry is pumpedinto the well, at a pressure sufficient to create fractures in the rockformation containing the petroleum. Once the fractures are created, thepressure is released and the proppant remains in the fractures andserves to maintain the fractures in an open state, or, in other words,to “prop” open the fractures. Proppants in use today are generally sandgrains or high strength, spherical pellets that create a highlypermeable pathway through which the petroleum can flow more readily.

[0006] Known proppants include natural materials such as sand orsynthetic materials such as glass, metal or ceramic pellets, usuallyclose to spherical in shape. Such known proppants have associatedproblems. For example, although sand is quite inexpensive and availablein large quantities, its function is impeded in situations where thereservoir is deep. In such cases, the tremendous pressures (for example8000 psi) cause the sand to be crushed and agglomerate, thus sealing thefractures. In effect, the sand no longer serves its intended function.This problem is also encountered when using some synthetic proppantssuch as glass beads. Synthetic proppants made from ceramics areconsiderably stronger than sand and glass and are able to withstand veryhigh pressures without crushing. However, ceramic proppants areconsiderably more expensive than sand and the like, thereby resulting inhigh production costs.

[0007] U.S. Pat. No. 4,547,468 and RE 34,371 teach known syntheticproppants that have the deficiencies as indicated above. Further U.S.Pat. No. 6,364,018 teaches the use of crushed nut shells and woodparticles as a proppant material.

[0008] It is also commonly known to provide the proppant material with asurface coating comprising a resinous material. Such coatings serve toprovide the proppant with a durable surface that is capable ofwithstanding the mechanical and chemical stresses that are associatedwith the fracturing process. Examples of such coatings are discussed inpublished Canadian patent application 2,198,812.

[0009] In petroleum production, it is also known that sulphur is acommon contaminant of the produced petroleum that must be extractedbefore the petroleum can be transported or used. Generally, petroleumproduction companies are forced to store the sulphur extracted from theproduced petroleum due to the low demand for such product or otherwisedispose of such product. Storage of the extracted sulphur is complicatedby the flammability of such product. Therefore, in either case, thepresence of sulphur compounds in the produced petroleum proves to be anextra cost for petroleum production companies.

[0010] It is an object of the present invention to provide a proppantthat overcomes deficiencies known in the art.

SUMMARY OF THE INVENTION

[0011] In one embodiment, the present invention provides a proppant forenhancing recovery of petroleum from rock formations comprising pelletsof a mixture of sulphur and a first binding and strengthening additive.

[0012] In another embodiment, the invention provides a process forforming a proppant for enhancing recovery of petroleum from rockformations, the proppant comprising pellets of a mixture of sulphur anda binding and strengthening additive, the process comprising:

[0013] heating said sulphur to form a liquid;

[0014] forming a mixture of liquid sulphur and the binding andstrengthening additive;

[0015] heating the mixture to maintain said mixture in a liquid state;

[0016] cooling and pelletizing the mixture;

[0017] retrieving and drying said pellets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018] These and other features of the preferred embodiments of theinvention will become more apparent in the following detaileddescription in which reference is made to the appended drawings wherein:

[0019]FIG. 1 is a schematic diagram of a manufacturing process forproppants according to an embodiment of the invention.

[0020]FIGS. 2a and 2 b are front and side view photographs of a proppantparticle comprising sand.

[0021]FIGS. 3a and 3 b are front and side view photographs of a proppantparticle comprising a ceramic material.

[0022]FIGS. 4a and 4 b are front and side view photographs of a proppantparticle comprising a sulphur material of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] The present invention provides a proppant for a hydraulicfracturing process that comprises generally spherical pellets made fromsulphur and cement as well as a method of forming such pellets. Thesulphur used in forming the proppant can be derived from the sulphurextracted from the petroleum produced from nearby wells. In the usualcase, such sulphur is generally disposed of with a high associated cost.Therefore, by utilizing such by-product to form the proppant, thesedisposal costs are avoided. In the preferred embodiment, the sulphurused in the present invention is obtained from petroleum during thepetroleum refining process. In this manner, the raw material for theproppants of the invention is readily obtained and, in fact, is aby-product of the petroleum extraction process. In the preferredembodiment, the sulphur used in the process of the present invention isin a liquid or elemental form.

[0024] It will be understood that the term “petroleum” as used hereinincludes oil, gas and any other hydrocarbon material that is extractedfrom subterranean accumulations.

[0025] A preferred process for manufacturing the proppant pellets of thepresent invention involves first mixing the liquid sulphur with a cementpowder. Preferably, the cement powder is sulphate resistant and isprovided to achieve a proportion of 3 parts sulphur to 1 part cementpowder (by weight) in the mixture. It will be understood that variousother proportions of the sulphur and cement components are possible. Themixture is heated to about 118° C. to melt the sulphur and is agitatedmechanically. This temperature corresponds to the melting point ofsulphur at atmospheric pressure. Other temperatures to achieve thisresult will be apparent to persons skilled in the art depending upon therelevant environmental conditions. The mixture is then dripped into acooling chamber containing cold water. In a preferred embodiment, thedripping is achieved by passing the mixture through a sieve plate or thelike. Upon contact with the cold water, the sulphur/cement mixture issolidified into pellets. The pellets can then be separated by drainingoff the water. Following this, the pellets are passed through a tumbledryer where they are allowed to dry and cure under heat (for example 65°C.). The formed pellets are then subjected to a screening process tosize the pellets.

[0026] The cement used in the above process serves as a binder andstrengthening agent. It will be understood that other binders andstrengthening agents, such as silicone and the like, can also be used inthe present invention.

[0027] In the preferred embodiment, the pellets, thus formed, are thencoated with an epoxy material such as Chem-Rez™.

[0028]FIG. 1 illustrates, schematically, a system 10 for forming thepellets. As shown, the sulphur separated from the petroleum stream isplaced in a tank 12. Tank 12 includes an agitator 14 and is heated. Acement powder, which is preferably sulphate resistant, is then added tothe tank. The cement powder (preferably type 50, sulphate resistantcement) is contained in a holding tank or hopper 16 and is provided intothe tank 12 through piping 18. The mixture of the sulphur and cementpowder is heated to about 118° C. while being agitated.

[0029] In one embodiment other additives such as silicone 19 can also beadded to the mixture.

[0030] The heated mixture is then pumped via pump 20 to a feed tank 22through piping 24. The feed tank 22 is provided with a plurality ofapertures 26 on its base 28. The sulphur/cement mixture is then allowedto drip through the holes into a cooling chamber 30. It will beunderstood that the diameter of the apertures 26 should not be largeenough to form a stream of the mixture but only drops. Further, it willbe understood that the diameter of the apertures 26 will also affect thesize of the drops formed and, therefore, the size (i.e. diameter) of thepellets. Further, the pellet size will also depend upon the compositionof the mixture.

[0031] Cooling chamber 30 is supplied with a spray of cold water througha plurality of ports 32 located on the side walls of the chamber 30. Thewater is allowed to circulate or swirl within the chamber 30. As thedrops 34 of the sulphur/cement mixture, still in a heated liquid state,enter into the cooling chamber 30 and contact the cooling water, theyare solidified into pellets 36. As illustrated, the cooling chamber 30is preferably vertically elongated so as to maximize the exposure of thesulphur/cement mixture to the cooling water, thereby ensuring adequatesolidification and proper formation of the pellet-like shape.

[0032] Upon solidification, the sulphur pellets drop to the bottom ofcooling chamber 30 due to gravity. A siphon 38 is used to extract theformed pellets 36 from the bottom of chamber 30. The pellets along withsome water are moved to a dewatering station 40. The station 40 includesa dewatering screen 42 onto which the pellets and water are deposited.The screen 42 serves to drain water while retaining the pellets 36. Thepellets are collected on a tray 44. The drained water is collected in atank 46, which includes a divider 48 that separates the collecting tank46 into two sections. The first section 50 is where the water from thedewatering station 40 is first deposited. The water, which stillincludes some fine pellets that may have passed through the dewateringscreen 42, is then pumped via pump 52 to a cyclone separator 54, whichseparates the second sample of solid pellets 36 a from the water andcollects them on a tray 56. The water is then transported to the secondsection 58 of the collecting tank 46. The water is then preferablypumped via pump 60, and through piping 61, back to the cooling chamber30 after cooling if necessary.

[0033] The pellets 36 and 36 a are then collected from trays 44 and 56and dried in a dryer (not shown). The dryer also allows the pellets tocure and is preferably set at a temperature of about 65° C. Followingthe drying step, the pellets can then be separated into the desired sizegroups using known screening methods. The sized pellets are then storedfor later use.

[0034] In a preferred embodiment, the pellets are subsequently coatedwith any known coating including, for example, an epoxy material. Suchcoating serves to increase the strength of the pellet and to provide abarrier to degradation or corrosion due to the presence of harshchemicals in the well. Further, the coating provides a smooth outersurface for the pellets, which enhances their flow properties and toreduce their abrasive effects on the pumps and piping used to convey thepellets. As known in the art, the physical properties of the coating canbe adjusted by varying the resin/hardener ratio.

[0035] Proppants according to the present invention can be used ascommonly known in the art. That is, the proppants of the presentinvention are first mixed with a fracturing fluid to form a slurry orsuspension that is then pumped under pressure through the well-head andinto the rock formation containing the petroleum to be extracted. Thehigh pressure fluid creates fractures in the rock and creates highlypermeable pathways through which the petroleum can flow. The proppant iscarried into the fractures by the fluid and is then left in thefractures thereby propping them open.

[0036] The use of sulphur in forming the proppant according to thepresent invention offers various advantages. Firstly, as discussedabove, the invention provides a viable alternative to expensive disposalof the sulphur extracted from the produced petroleum. Furthermore, thespecific gravity of sulphur is such that it is well suited to work withfracturing fluids.

[0037] The process of the present invention also offers variousadvantages. For example, unlike other proppants such as sand, theinvention provides a process whereby the size of the proppant particlesis highly controlled. Further, unlike sand or ceramic materials,proppants of the present invention are manufactured to highly sphericalgeometries, which enhances their flow properties as well as porositywhen positioned inside a fracture. FIGS. 2 to 4 illustrate the particlegeometries for sand and ceramic proppants as well as that for proppantsof the present invention. As can be seen, the proppant particlesaccording to the present invention are considerably more spherical inshape. In addition, the surface coating of the particles offers a meansof controlling the hardness of the pellets, thereby allowing theformation of pellets that are particularly suited for specific depthranges.

[0038] Although the invention has been described with reference tocertain specific embodiments, various modifications thereof will beapparent to those skilled in the art without departing from the spiritand scope of the invention as outlined in the claims appended hereto.

The embodiments of the invention in which an exclusive property orprivilege is claimed are defined as follows:
 1. A proppant for enhancingrecovery of petroleum from rock formations comprising pellets of amixture of sulphur and a first binding and strengthening additive. 2.The process of claim 1 wherein the first binding and strengtheningadditive is a cement powder.
 3. The process of claim 1 wherein thesulphur is extracted from petroleum during petroleum refining processes.4. The proppant of claim 1 wherein the weight ratio of said sulphur andsaid additive is 3:1.
 5. The proppant of claim 1 wherein the pelletsinclude a surface coating.
 6. The proppant of claim 5 wherein thesurface coating comprises a smooth, durable material.
 7. The proppant ofclaim 6 wherein the surface coating is comprised of a resin
 8. Theproppant of claim 6 wherein the surface coating comprises an epoxy. 9.The proppant of claim 1 further including a second binding additive. 10.The proppant of claim 9 wherein said second binding additive issilicone.
 11. A process for forming a proppant for enhancing recovery ofpetroleum from rock formations, the proppant comprising pellets of amixture of sulphur and a binding and strengthening additive, the processcomprising: heating said sulphur to form a liquid; forming a mixture ofliquid sulphur and the binding and strengthening additive; heating themixture to maintain said mixture in a liquid state; cooling andpelletizing the mixture; retrieving and drying said pellets.
 12. Theprocess of claim 11 wherein said mixture is heated to a temperature ator above the melting point of the sulphur component.
 13. The process ofclaim 12 wherein said mixture is heated to about 118° C.
 14. The processof claim 11 wherein said pelletizing step includes dripping said heatedliquid mixture into a cooling bath to solidify said pellets.
 15. Theprocess of claim 14 wherein said cooling bath comprises a chamber cooledwith swirling cold water.
 16. The process of claim 11 further includingsorting said pellets by size.
 17. The process of claim 11 furtherincluding coating said pellets with a surface coating.
 18. The processof claim 17 wherein said coating comprises a resin material.
 19. Theprocess of claim 11 further comprising extracting said sulphur frompetroleum in a petroleum refining process.